calcSumProb {graph} | R Documentation

## Calculate the probability that a subgraph has an unusual number of edges.

### Description

For any graph a set of nodes can be used to obtain an induced subgraph (see `subGraph`). An interesting question is whether that subgraph has an unusually large number of edges. This function computes the probability that a random subgraph with the same number of nodes has more edges than the number observed in the presented subgraph. The appropriate probability distribution is the hypergeometric.

### Usage

    calcSumProb(sg, g)

### Arguments

`sg` | subgraph made from the original graph
`g` | original graph object from which the subgraph was made

### Details

The computation is based on the following argument. In the original graph there are n nodes and hence N=n*(n-1)/2 edges in the complete graph. If we consider these N nodes to be of two types, corresponding to those that are either in our graph, `g`, or not in it. Then we think of the subgraph which has say m nodes and M=m*(m-1)/2 possible edges as representing M draws from an urn containing N balls of which some are white (those in `g`) and some are black. We count the number of edges in the subgraph and use a Hypergeomtric distribution to ask whether our subgraph is particularly dense.

### Value

The probability of having greater than or equal to the subgraph's number of edges is returned.

### Author(s)

Elizabeth Whalen

### See Also

`calcProb`

### Examples

      set.seed(123)
      V <- letters[14:22]
      g1 <- randomEGraph(V, .2)

      sg1 <- subGraph(letters[c(15,17,20,21,22)], g1)
      calcSumProb(sg1, g1)

[Package graph version 1.66.0 Index]